Gas sensor and nitrogen oxide sensor

ABSTRACT

A gas sensor for measuring an amount of a measurement gas component, including a solid electrolyte having an internal space, a gas-introducing port for introducing measurement gas from an external space into the internal space, diffusion rate-determining means between the internal space and the gas-introducing port, and inner and outer pumping electrodes for pumping-processing oxygen contained in the measurement gas. The diffusion rate-determining means includes slits each having, when viewed in a plane substantially perpendicular to a longitudinal extension axis thereof, two dimensions, with at least one dimension of each slit being not more than 10 microns.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. application Ser. No.10/342,846, filed Jan. 15, 2003, which is a continuation of U.S.application Ser. No. 10/021,196, filed Oct. 30, 2001, now U.S. Pat. No.6,527,929, which is a continuation of U.S. application Ser. No.09/348,857, filed Jul. 7, 1999, now U.S. Pat. No. 6,355,152, theentireties of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a gas sensor and a nitrogenoxide sensor for measuring oxides such as O₂, NO, NO₂, SO₂, CO₂, and H₂Ocontained in, for example, atmospheric air and exhaust gas dischargedfrom vehicles or automobiles, and inflammable gases such as CO and CnHm.

[0004] 2. Description of the Related Art

[0005] Those hitherto known as the method for measuring NOx in ameasurement gas such as combustion gas include a technique in which theNOx-reducing ability of Rh is utilized to use a sensor comprising a Ptelectrode and an Rh electrode formed on an oxygen ion-conductive solidelectrolyte such as zirconia so that an electromotive force generatedbetween the both electrodes is measured.

[0006] However, the sensor as described above suffers the followingproblems. That is, the electromotive force is greatly changed dependingon the change in concentration of oxygen contained in the combustion gasas the measurement gas. Moreover, the change in electromotive force issmall with respect to the change in concentration of NOx. For thisreason, the conventional sensor tends to suffer influence of noise.

[0007] Further, in order to bring out the NOx-reducing ability, it isindispensable to use a reducing gas such as CO. For this reason, theamount of produced CO is generally smaller than the amount of producedNOx under a lean fuel combustion condition in which a large amount ofNOx is produced. Therefore, the conventional sensor has a drawback inthat it is impossible to perform the measurement for a combustion gasproduced under such a combustion condition.

[0008] In order to solve the problems as described above, for example,Japanese Laid-Open Patent Publication No. 8-271476 discloses a NOxsensor comprising pumping electrodes having different NOx-decomposingabilities arranged in a first internal space which communicates with ameasurement gas-existing space and in a second internal space whichcommunicates with the first internal space, and a method for measuringthe NOx concentration in which the O₂ concentration is adjusted by usinga first pumping cell arranged in the first internal space, and NO isdecomposed by using a decomposing pump arranged in the second internalspace so that the NOx concentration is measured on the basis of apumping current flowing through the decomposing pump.

[0009] Further, Japanese Laid-Open Patent Publication No. 9-113484discloses a sensor element comprising an auxiliary pumping electrodearranged in a second internal space so that the oxygen concentration inthe second internal space is controlled to be constant even when theoxygen concentration is suddenly changed.

[0010] The following fact has been revealed when a gas sensor isattached to an exhaust system of an internal combustion engine such asan automobile engine, and the internal combustion engine is operated.That is, in ordinary cases, the sensor output ordinarily makesproportional change based on an anchoring point of zero in accordancewith the change in oxygen concentration as shown by a solid line “a” inFIG. 32. However, under a specified operation condition, the sensoroutput is subjected to shift-up as a whole as shown by a solid line “b”.

[0011] In general, as shown in FIG. 33, the total pressure of theexhaust gas discharged from the automobile engine is composed of aconstant static pressure and a dynamic pressure generated by thepulsation of the exhaust gas pressure. The fluctuation cycle of thedynamic pressure is synchronized with the explosion cycle of the engine.As a result of investigation on the cause of the shift-up of the sensoroutput, it has been revealed that the shift-up occurs when the pulsationamount (=dynamic pressure) of the exhaust gas pressure is large ascompared with the static pressure.

[0012] That is, as shown in FIG. 34, the shift amount of the sensoroutput has been measured with respect to the ratio between the dynamicpressure and the static pressure (dynamic pressure/static pressure). Asa result, the shift amount is approximately zero when the dynamicpressure/static pressure is not more than about 25%. However, the shiftamount increases proportionally from the stage at which the dynamicpressure/static pressure exceeds about 25%.

[0013] Therefore, when the dynamic pressure is increased, it isinevitable to suffer any deterioration of the correlation between theoxygen-pumping amount effected by the main pump in the first space andthe oxygen concentration in the measurement gas. It is feared that thedisturbance of the oxygen concentration caused in the first space maybring about any deterioration concerning the control of the oxygenconcentration in the second space communicating with the first space andthe accuracy of measurement effected by the detecting electrode as theNOx-detecting section.

SUMMARY OF THE INVENTION

[0014] The present invention has been made taking such problems intoconsideration, an object of which is to provide a gas sensor and anitrogen oxide sensor which make it possible to avoid the influence ofthe pulsation of the exhaust gas pressure generated in the measurementgas, and improve the measurement accuracy obtained on the detectingelectrode.

[0015] According to the present invention, there is provided a gassensor for measuring an amount of a measurement gas component containedin a measurement gas existing in external space; the gas sensor at leastcomprising a substrate composed of a solid electrolyte to make contactwith the external space; an internal space formed at the inside of thesubstrate; a diffusion rate-determining means formed with a slit forintroducing the measurement gas from the external space via agas-introducing port under a predetermined diffusion resistance; and apumping means including an inner pumping electrode and an outer pumpingelectrode formed at the inside and outside of the internal spacerespectively, for pumping-processing oxygen contained in the measurementgas introduced from the external space, on the basis of a controlvoltage applied between the electrodes; wherein a dimension of a certainfactor for forming a cross-sectional configuration of the diffusionrate-determining means is not more than 10 μm.

[0016] The limiting current value Ip in the pumping means isapproximated by the following theoretical expression for the limitingcurrent.

Ip≈(4F/RT)×D×(S/L)×(POe−POd)

[0017] In the expression, F represents the Faraday constant (=96500A/sec), R represents the gas constant (=82.05 cm³·atm/mol·K), Trepresents the absolute temperature (K), D represents the diffusioncoefficient (cm²/sec), S represents the cross-sectional area (cm²) ofthe diffusion rate-determining means, L represents the passage length(cm) of the diffusion rate-determining means, POe represents the partialpressure of oxygen (atm) at the outside of the diffusionrate-determining means, and POd represents the partial pressure ofoxygen (atm) at the inside of the diffusion rate-determining means.

[0018] The present invention defines the factor for forming thecross-sectional area S of the diffusion rate-determining means in thetheoretical limiting current expression. Especially, it is defined thatthe certain factor of the dimension for forming the cross-sectional areaS is not more than 10 μm.

[0019] In this arrangement, the pulsation (=dynamic pressure) of theexhaust gas pressure is attenuated by the wall resistance of thediffusion rate-determining means. Specifically, the attenuation iseffected up to the level at which the ratio between the dynamic pressureand the static pressure (dynamic pressure/static pressure) is not morethan 25%. Therefore, it is possible to effectively suppress the shift-upphenomenon of the sensor output which would be otherwise caused by thefluctuation of the dynamic pressure.

[0020] It is also preferable for the gas sensor constructed as describedabove that when the cross-sectional configuration of the diffusionrate-determining means is formed with at least one lateral type slit,the certain factor is a length of the slit in a vertical direction.Alternatively, it is also preferable that when the cross-sectionalconfiguration of the diffusion rate-determining means is formed with atleast one vertical type slit, the certain factor is a length of the slitin a lateral direction.

[0021] It is also preferable for the gas sensor constructed as describedabove that a buffering space is provided between the gas-introducingport and the diffusion rate-determining means. Usually, the oxygensuddenly enters the sensor element via the gas-introducing port due tothe pulsation of the exhaust gas pressure brought about in the externalspace. However, in this arrangement, the oxygen from the external spacedoes not enter the processing space directly, but it enters thebuffering space disposed at the upstream stage thereof. In other words,the sudden change in oxygen concentration, which is caused by thepulsation of the exhaust gas pressure, is counteracted by the bufferingspace. Thus, the influence of the pulsation of the exhaust gas pressure,which is exerted on the internal space, is in an almost negligibledegree.

[0022] As a result, the correlation is improved between theoxygen-pumping amount effected by the pumping means in the processingspace and the oxygen concentration in the measurement gas. It ispossible to improve the measurement accuracy obtained on the measuringpumping means or the concentration-detecting means. Simultaneously, forexample, it is possible to concurrently use the internal space as asensor for determining the air-fuel ratio.

[0023] It is also preferable for the gas sensor constructed as describedabove that a clogging-preventive section and the buffering space areprovided in series between the gas-introducing port and the internalspace (processing space); a front aperture of the clogging-preventivesection is used to form the gas-introducing port; and a diffusionrate-determining section for giving a predetermined diffusion resistanceto the measurement gas is provided between the clogging-preventivesection and the buffering space.

[0024] In this arrangement, the gas sensor is prevented from clogging ofparticles (for example, soot and oil combustion waste) produced in themeasurement gas in the external space, which would be otherwise causedin the vicinity of the inlet of the buffering space. Thus, it ispossible to measure the predetermined gas component more accurately.Further, it is possible to maintain a highly accurate state over a longperiod of time.

[0025] It is also preferable for the gas sensor constructed as describedabove that the oxygen contained in the measurement gas introduced fromthe external space into the internal space is pumping-processed by usingthe pumping means to make control so that a partial pressure of oxygenin the internal space (processing space) has a predetermined value atwhich a predetermined gas component in the measurement gas is notdecomposable.

[0026] It is also preferable that the gas sensor further comprises ameasuring pumping means for decomposing the predetermined gas componentcontained in the measurement gas after being pumping-processed by thepumping means, by means of catalytic action and/or electrolysis, andpumping processing oxygen produced by the decomposition; wherein thepredetermined gas component contained in the measurement gas is measuredon the basis of a pumping current flowing through the measuring pumpingmeans in accordance with the pumping process effected by the measuringpumping means.

[0027] Alternatively, it is also preferable that the gas sensor furthercomprises an oxygen partial pressure-detecting means for decomposing thepredetermined gas component contained in the measurement gas after beingpumping-processed by the pumping means, by means of catalytic action,and generating an electromotive force corresponding to a differencebetween an amount of oxygen produced by the decomposition and an amountof oxygen contained in a reference gas; wherein the predetermined gascomponent contained in the measurement gas is measured on the basis ofthe electromotive force detected by the oxygen partialpressure-detecting means.

[0028] According to another aspect of the present invention, there isprovided a nitrogen oxide sensor for measuring an amount of a nitrogenoxide component contained in a measurement gas existing in externalspace; the nitrogen oxide sensor at least comprising a substratecomposed of an oxygen ion-conductive solid electrolyte to make contactwith the external space; a first internal space formed at the inside ofthe substrate and communicating with the external space; a firstdiffusion rate-determining means formed with a slit for introducing themeasurement gas into the first internal space under a predetermineddiffusion resistance; a main pumping means including a first innerpumping electrode and a first outer pumping electrode formed at theinside and outside of the first internal space respectively, forpumping-processing oxygen contained in the measurement gas introducedfrom the external space, on the basis of a control voltage appliedbetween the electrodes so that a partial pressure of oxygen in the firstinternal space is controlled to have a predetermined value at which NOis not substantially decomposable; a second internal space communicatingwith the first internal space; a second diffusion rate-determining meansformed with a slit for introducing an atmosphere pumping-processed inthe first internal space into the second internal space under apredetermined diffusion resistance; and a measuring pumping meansincluding a second inner pumping electrode and a second outer pumpingelectrode formed at the inside and outside of the second internal spacerespectively, for decomposing NO contained in the atmosphere introducedfrom the first internal space, by means of catalytic action and/orelectrolysis to pumping-process oxygen produced by the decomposition;wherein the amount of nitrogen oxide contained in the measurement gas ismeasured on the basis of a pumping current flowing through the measuringpumping means in accordance with the pumping process effected by themeasuring pumping means; and a dimension of a certain factor for forminga cross-sectional configuration of at least one of the diffusionrate-determining means is not more than 10 μm.

[0029] According to the present invention, the pulsation (=dynamicpressure) of the exhaust gas pressure is attenuated by the wallresistance of the diffusion rate-determining means. Specifically, theattenuation is effected up to the level at which the ratio between thedynamic pressure and the static pressure (dynamic pressure/staticpressure) is not more than 25%. Therefore, it is possible to effectivelysuppress the shift-up phenomenon of the sensor output which would beotherwise caused by the fluctuation of the dynamic pressure.

[0030] According to still another aspect of the present invention, thereis provided a nitrogen oxide sensor for measuring an amount of anitrogen oxide component contained in a measurement gas existing inexternal space; the nitrogen oxide sensor at least comprising asubstrate composed of an oxygen ion-conductive solid electrolyte to makecontact with the external space; a first internal space formed at theinside of the substrate and communicating with the external space; afirst diffusion rate-determining means formed with a slit forintroducing the measurement gas into the first internal space under apredetermined diffusion resistance; a main pumping means including aninner pumping electrode and an outer pumping electrode formed at theinside and outside of the first internal space respectively, forpumping-processing oxygen contained in the measurement gas introducedfrom the external space, on the basis of a control voltage appliedbetween the electrodes so that a partial pressure of oxygen in the firstinternal space is controlled to have a predetermined value at which NOis not substantially decomposable; a second internal space communicatingwith the first internal space; a second diffusion rate-determining meansformed with a slit for introducing an atmosphere pumping-processed inthe first internal space into the second internal space under apredetermined diffusion resistance; and an oxygen partialpressure-detecting means including an inner measuring electrode and anouter measuring electrode formed at the inside and outside of the secondinternal space respectively, for decomposing NO contained in theatmosphere introduced from the first internal space, by means ofcatalytic action to generate an electromotive force corresponding to adifference between an amount of oxygen produced by the decomposition andan amount of oxygen contained in a reference gas; wherein the amount ofnitrogen oxide contained in the measurement gas is measured on the basisof the electromotive force detected by the oxygen partialpressure-detecting means; and a dimension of a certain factor forforming a cross-sectional configuration of at least one of the diffusionrate-determining means is not more than 10 μm.

[0031] Also in this aspect, the pulsation (=dynamic pressure) of theexhaust gas pressure is attenuated by the wall resistance of thediffusion rate-determining means. Specifically, the attenuation iseffected up to the level at which the ratio between the dynamic pressureand the static pressure (dynamic pressure/static pressure) is not morethan 25%. Therefore, it is possible to effectively suppress the shift-upphenomenon of the sensor output which would be otherwise caused by thefluctuation of the dynamic pressure.

[0032] The above and other objects, features, and advantages of thepresent invention will become more apparent from the followingdescription when taken in conjunction with the accompanying drawings inwhich a preferred embodiment of the present invention is shown by way ofillustrative example.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033]FIG. 1A shows a front view illustrating an arrangement of a gassensor according to a first embodiment;

[0034]FIG. 1B shows a plan view thereof;

[0035]FIG. 2 shows a sectional view taken along a line II-II shown inFIG. 1B;

[0036]FIG. 3A shows a front view illustrating an arrangement concerningWorking Example used in first and second illustrative experiments;

[0037]FIG. 3B shows a plan view thereof;

[0038]FIG. 4 shows a perspective view illustrating the arrangementconcerning Working Example used in the first and second illustrativeexperiments, especially illustrating extracted arrangements of first andsecond diffusion rate-determining sections;

[0039]FIG. 5A shows a front view illustrating an arrangement concerningComparative Example used in the first and second illustrativeexperiments;

[0040]FIG. 5B shows a plan view thereof;

[0041]FIG. 6 shows a perspective view illustrating the arrangementconcerning Comparative Example used in the first and second illustrativeexperiments, especially illustrating extracted arrangements of first andsecond diffusion rate-determining sections;

[0042]FIG. 7 shows the characteristic illustrating the change in sensoroutput with respect to the oxygen concentration, obtained when themeasurement condition is changed in Comparative Example;

[0043]FIG. 8 shows the characteristic illustrating the change in sensoroutput with respect to the NOx concentration, obtained when themeasurement condition is changed in Comparative Example;

[0044]FIG. 9 shows the characteristic illustrating the change in sensoroutput with respect to the oxygen concentration, obtained when themeasurement condition is changed in Working Example;

[0045]FIG. 10 shows the characteristic illustrating the change in sensoroutput with respect to the NOx concentration, obtained when themeasurement condition is changed in Working Example;

[0046]FIG. 11A shows a waveform illustrating the fluctuation of theexhaust gas pressure in the vicinity of a gas-introducing port inComparative Example;

[0047]FIG. 11B shows a waveform illustrating the fluctuation of theexhaust gas pressure in the vicinity of an inlet of a first chamber;

[0048]FIG. 12A shows a waveform illustrating the fluctuation of theexhaust gas pressure in the vicinity of a gas-introducing port inWorking Example;

[0049]FIG. 12B shows a waveform illustrating the fluctuation of theexhaust gas pressure in the vicinity of an inlet of a first chamber;

[0050]FIG. 13A shows a front view illustrating an arrangement of a gassensor according to a first modified embodiment;

[0051]FIG. 13B shows a front view thereof;

[0052]FIG. 14 shows a sectional view taken along a line XIV-XIV shown inFIG. 13B;

[0053]FIG. 15A shows a front view illustrating an arrangement of a gassensor according to a second modified embodiment;

[0054]FIG. 15B shows a front view thereof;

[0055]FIG. 16 shows a sectional view taken along a line XVI-XVI shown inFIG. 15B;

[0056]FIG. 17A shows a front view illustrating an arrangement of a gassensor according to a third modified embodiment;

[0057]FIG. 17B shows a front view thereof;

[0058]FIG. 18 shows a sectional view taken along a line XVIII-XVIIIshown in FIG. 17B;

[0059]FIG. 19A shows a front view illustrating an arrangement of a gassensor according to a fourth modified embodiment;

[0060]FIG. 19B shows a front view thereof;

[0061]FIG. 20 shows a sectional view taken along a line XX-XX shown inFIG. 19B;

[0062]FIG. 21A shows a front view illustrating an arrangement of a gassensor according to a fifth modified embodiment;

[0063]FIG. 21B shows a front view thereof;

[0064]FIG. 22 shows a sectional view taken along a line XXII-XXII shownin FIG. 21B;

[0065]FIG. 23A shows a front view illustrating an arrangement of a gassensor according to a sixth modified embodiment;

[0066]FIG. 23B shows a front view thereof;

[0067]FIG. 24 shows a sectional view taken along a line XXIV-XXIV shownin FIG. 23B;

[0068]FIG. 25A shows a front view illustrating an arrangement of a gassensor according to a seventh modified embodiment;

[0069]FIG. 25B shows a front view thereof;

[0070]FIG. 26 shows a sectional view taken along a line XXVI-XXVI shownin FIG. 25B;

[0071]FIG. 27A shows a front view illustrating an arrangement of a gassensor according to an eighth modified embodiment;

[0072]FIG. 27B shows a front view thereof;

[0073]FIG. 28 shows a sectional view taken along a line XXVIII-XXVIIIshown in FIG. 27B;

[0074]FIG. 29A shows a front view illustrating an arrangement of a gassensor according to a ninth modified embodiment;

[0075]FIG. 29B shows a front view thereof;

[0076]FIG. 30 shows a sectional view taken along a line XXX-XXX shown inFIG. 29B;

[0077]FIG. 31 shows a sectional view illustrating an arrangement of agas sensor according to a second embodiment;

[0078]FIG. 32 shows the characteristic illustrating the change in sensoroutput with respect to the oxygen concentration concerning theconventional gas sensor;

[0079]FIG. 33 illustrates the total pressure of the exhaust gasdischarged from the automobile engine; and

[0080]FIG. 34 shows the characteristic illustrating the shift amount ofthe sensor output with respect to the ratio between the dynamic pressureand the static pressure (dynamic pressure/static pressure).

DETAILED DESCRIPTION OF THE INVENTION

[0081] Explanation will be made below with reference to FIGS. 1A to 31for several illustrative embodiments in which the gas sensor accordingto the present invention is applied to gas sensors for measuring oxidessuch as O₂, NO, NO₂, SO₂, CO₂, and H₂O contained in, for example,atmospheric air and exhaust gas discharged from vehicles or automobiles,and inflammable gases such as CO and CnHm.

[0082] As shown in FIGS. 1A, 1B, and 2, a gas sensor 10A according tothe first embodiment includes a sensor element 14 provided with asubstrate comprising, for example, six stacked solid electrolyte layers12 a to 12 f composed of ceramics based on the use of oxygenion-conductive solid electrolytes such as ZrO₂.

[0083] In the sensor element 14, first and second layers from the bottomare designated as first and second substrate layers 12 a, 12 brespectively. Third and fifth layers from the bottom are designated asfirst and second spacer layers 12 c, 12 e respectively. Fourth and sixthlayers from the bottom are designated as first and second solidelectrolyte layers 12 d, 12 f respectively.

[0084] A space 16 (reference gas-introducing space 16), into which areference gas such as atmospheric air to be used as a reference formeasuring oxides is introduced, is formed between the second substratelayer 12 b and the first solid electrolyte layer 12 d, the space 16being comparted by a lower surface of the first solid electrolyte layer12 d, an upper surface of the second substrate layer 12 b, and sidesurfaces of the first spacer layer 12 c.

[0085] A first chamber 18 for adjusting the partial pressure of oxygenin a measurement gas is formed and comparted between a lower surface ofthe second solid electrolyte layer 12 f and an upper surface of thefirst solid electrolyte layer 12 d. A second chamber 20 for finelyadjusting the partial pressure of oxygen in the measurement gas andmeasuring oxides, for example, nitrogen oxides (NOx) in the measurementgas is formed and comparted between the lower surface of the secondsolid electrolyte layer 12 f and the upper surface of the first solidelectrolyte layer 12 d.

[0086] A gas-introducing port 22, which is formed at the forward end ofthe sensor element 14, communicates with the first chamber 18 via afirst diffusion rate-determining section 26. The first chamber 18communicates with the second chamber 20 via a second diffusionrate-determining section 28.

[0087] In this embodiment, each of the first and second diffusionrate-determining sections 26, 28 gives a predetermined diffusionresistance to the measurement gas to be introduced into the firstchamber 18 and the second chamber 20 respectively. As shown in FIG. 1A,the first diffusion rate-determining section 26 is formed by twolaterally extending slits 30, 32. Specifically, the first diffusionrate-determining section 26 includes the slit 30 having a laterallyextending aperture formed at a front end portion of the second spacerlayer 12 e to make contact with the lower surface of the second solidelectrolyte layer 12 f, the aperture being formed to extend with anidentical aperture width up to the first chamber 18. The first diffusionrate-determining section 26 also includes the slit 32 having a laterallyextending aperture formed at a front end portion of the second spacerlayer 12 e to make contact with the upper surface of the first solidelectrolyte layer 12 d, the aperture being formed to extend with anidentical aperture width up to the first chamber 18.

[0088] In the first embodiment, the respective slits 30, 32 haveapproximately the same cross-sectional configuration. As shown in FIG.1A, the length ta in the vertical direction is not more than 10 μm, andthe length tb in the lateral direction is about 2 mm.

[0089] The second diffusion rate-determining section 28 is also formedwith two laterally extending slits 34, 36 each having a cross-sectionalconfiguration similar to those of the first diffusion rate-determiningsection 26. It is allowable that a porous member composed of ZrO₂ or thelike is arranged and packed in each of the slits 34, 36 of the seconddiffusion rate-determining section 28 so that the diffusion resistanceof the second diffusion rate-determining section 28 is larger than thediffusion resistance of the first diffusion rate-determining section 26.It is preferable that the diffusion resistance of the second diffusionrate-determining section 28 is larger than that of the first diffusionrate-determining section 26. However, no problem occurs even when theformer is smaller than the latter.

[0090] The atmosphere in the first chamber 18 is introduced into thesecond chamber 20 under the predetermined diffusion resistance via thesecond diffusion rate-determining section 28.

[0091] An inner pumping electrode 40 having a substantially rectangularplanar configuration and composed of a porous cermet electrode (forexample, a cermet electrode of Pt.ZrO₂ containing 1% Au) is formed onthe entire lower surface portion for forming the first chamber 18, ofthe lower surface of the second solid electrolyte layer 12 f. An outerpumping electrode 42 is formed on a portion corresponding to the innerpumping electrode 40, of the upper surface of the second solidelectrolyte layer 12 f. An electrochemical pumping cell, i.e., a mainpumping cell 44 is constructed by the inner pumping electrode 40, theouter pumping electrode 42, and the second solid electrolyte layer 12 finterposed between the both electrodes 40, 42.

[0092] A desired control voltage (pumping voltage) Vp1 is appliedbetween the inner pumping electrode 40 and the outer pumping electrode42 of the main pumping cell 44 by the aid of an external variable powersource 46 to allow a pumping current Ip1 to flow in a positive ornegative direction between the outer pumping electrode 42 and the innerpumping electrode 40. Thus, the oxygen in the atmosphere in the firstchamber 18 can be pumped out to the external space, or the oxygen in theexternal space can be pumped into the first chamber 18.

[0093] A reference electrode 48 is formed on a lower surface portionexposed to the reference gas-introducing space 16, of the lower surfaceof the first solid electrolyte layer 12 d. An electrochemical sensorcell, i.e., a controlling oxygen partial pressure-detecting cell 50 isconstructed by the inner pumping electrode 40, the reference electrode48, the second solid electrolyte layer 12 f, the second spacer layer 12e, and the first solid electrolyte layer 12 d.

[0094] The controlling oxygen partial pressure-detecting cell 50 isoperated as follows. That is, an electromotive force V1 is generatedbetween the inner pumping electrode 40 and the reference electrode 48 onthe basis of a difference in oxygen concentration between the atmospherein the first chamber 18 and the reference gas (atmospheric air) in thereference gas-introducing space 16. The partial pressure of oxygen inthe atmosphere in the first chamber 18 can be detected by using theelectromotive force V1.

[0095] The detected value of the partial pressure of oxygen is used tofeedback-control the variable power source 46. Specifically, the pumpingoperation effected by the main pumping cell 44 is controlled by the aidof a feedback control system 52 for the main pump so that the partialpressure of oxygen in the atmosphere in the first chamber 18 has apredetermined value which is sufficiently low to control the partialpressure of oxygen in the second chamber 20 in the next step.

[0096] The feedback control system 52 comprises a circuit constructed tofeedback-control the pumping current Vp1 between the outer pumpingelectrode 42 and the inner pumping electrode 40 so that a difference(detection voltage V1) between an electric potential of the innerpumping electrode 40 and an electric potential of the referenceelectrode 48 is at a predetermined voltage level. In this embodiment,the inner pumping electrode 40 is grounded.

[0097] Therefore, the main pumping cell 44 pumps out or pumps in oxygenin an amount corresponding to the level of the pumping voltage Vp1, ofthe measurement gas introduced into the first chamber 18. The oxygenconcentration in the first chamber 18 is subjected to feedback controlto give a predetermined level by repeating the series of operationsdescribed above. In this state, the pumping current Ip1, which flowsbetween the outer pumping electrode 42 and the inner pumping electrode40, indicates the difference between the oxygen concentration in themeasurement gas and the controlled oxygen concentration in the firstchamber 18. The pumping current Ip1 can be used to measure the oxygenconcentration in the measurement gas.

[0098] Each of the inner pumping electrode 40 and the outer pumpingelectrode 42 is composed of a porous cermet electrode which is made of ametal such as Pt and ceramics such as ZrO₂. It is necessary to use amaterial which has a weak reducing ability or no reducing ability withrespect to the NO component in the measurement gas, for the innerpumping electrode 40 disposed in the first chamber 18 to make contactwith the measurement gas. It is preferable that the inner pumpingelectrode 40 is composed of, for example, a compound having theperovskite structure such as La₃CuO₄, a cermet comprising ceramics and ametal such as Au having a low catalytic activity, or a cermet comprisingceramics, a metal of the Pt group, and a metal such as Au having a lowcatalytic activity. When an alloy composed of Au and a metal of the Ptgroup is used as an electrode material, it is preferable to add Au in anamount of 0.03 to 35% by volume of the entire metal component.

[0099] In the gas sensor 10A according to the first embodiment, adetecting electrode 60 having a substantially rectangular planarconfiguration and composed of a porous cermet electrode is formed at aportion separated from the second diffusion rate-determining section 28,on an upper surface portion for forming the second chamber 20, of theupper surface of the first solid electrolyte layer 12 d. An alumina filmfor constructing a third diffusion rate-determining section 62 is formedso that the detecting electrode 60 is covered therewith. Anelectrochemical pumping cell, i.e., a measuring pumping cell 64 isconstructed by the detecting electrode 60, the reference electrode 48,and the first solid electrolyte layer 12 d.

[0100] The detecting electrode 60 is composed of a porous cermetcomprising zirconia as ceramics and a metal capable of reducing NOx asthe measurement gas component. Accordingly, the detecting electrode 60functions as a NOx-reducing catalyst for reducing NOx existing in theatmosphere in the second chamber 20. Further, the oxygen in theatmosphere in the second chamber 20 can be pumped out to the referencegas-introducing space 16 by applying a constant voltage Vp2 between thedetecting electrode 60 and the reference electrode 48 by the aid of a DCpower source 66. The pumping current Ip2, which is allowed to flow inaccordance with the pumping operation performed by the measuring pumpingcell 64, is detected by an ammeter 68.

[0101] The constant voltage (DC) power source 66 can apply a voltage ofa magnitude to give a limiting current to the pumping for oxygenproduced during decomposition in the measuring pumping cell 64 under theinflow of NOx restricted by the third diffusion rate-determining section62.

[0102] On the other hand, an auxiliary pumping electrode 70 having asubstantially rectangular planar configuration and composed of a porouscermet electrode (for example, a cermet electrode of Pt.ZrO₂ containing1% Au) is formed on the entire lower surface portion for forming thesecond chamber 20, of the lower surface of the second solid electrolytelayer 12 f. An auxiliary electrochemical pumping cell, i.e., anauxiliary pumping cell 72 is constructed by the auxiliary pumpingelectrode 70, the second solid electrolyte layer 12 f, the second spacerlayer 12 e, the first solid electrolyte layer 12 d, and the referenceelectrode 48.

[0103] The auxiliary pumping electrode 70 is based on the use of amaterial having a weak reducing ability or no reducing ability withrespect to the NO component contained in the measurement gas, in thesame manner as in the inner pumping electrode 40 of the main pumpingcell 44. In this embodiment, it is preferable that the auxiliary pumpingelectrode 70 is composed of, for example, a compound having theperovskite structure such as La₃CuO₄, a cermet comprising ceramics and ametal having a low catalytic activity such as Au, or a cermet comprisingceramics, a metal of the Pt group, and a metal having a low catalyticactivity such as Au. Further, when an alloy comprising Au and a metal ofthe Pt group is used as an electrode material, it is preferable to addAu in an amount of 0.03 to 35% by volume of the entire metal components.

[0104] A desired constant voltage Vp3 is applied between the referenceelectrode 48 and the auxiliary pumping electrode 70 of the auxiliarypumping cell 72 by the aid of an external DC power source 74. Thus, theoxygen in the atmosphere in the second chamber 20 can be pumped out tothe reference gas-introducing space 16.

[0105] Accordingly, the partial pressure of oxygen in the atmosphere inthe second chamber 20 is allowed to have a low value of partial pressureof oxygen at which the measurement of the amount of the objectivecomponent is not substantially affected, under the condition in whichthe measurement gas component (NOx) is not substantially reduced ordecomposed. In this embodiment, owing to the operation of the mainpumping cell 44 for the first chamber 18, the change in amount of oxygenintroduced into the second chamber 20 is greatly reduced as comparedwith the change in the measurement gas. Accordingly, the partialpressure of oxygen in the second chamber 20 is accurately controlled tobe constant.

[0106] Therefore, in the gas sensor 10A according to the firstembodiment constructed as described above, the measurement gas, forwhich the partial pressure of oxygen has been controlled in the secondchamber 20, is introduced into the detecting electrode 60.

[0107] As shown in FIG. 2, the gas sensor 10A according to the firstembodiment further comprises a heater 80 for generating heat inaccordance with electric power supply from the outside. The heater 80 isembedded in a form of being vertically interposed between the first andsecond substrate layers 12 a, 12 b. The heater 80 is provided in orderto increase the conductivity of oxygen ion. An insulative layer 82composed of alumina or the like is formed to cover upper and lowersurfaces of the heater 80 so that the heater 80 is electricallyinsulated from the first and second substrate layers 12 a, 12 b.

[0108] The heater 80 is arranged over the entire portion ranging fromthe first chamber 18 to the second chamber 20. Accordingly, each of thefirst chamber 18 and the second chamber 20 is heated to a predeterminedtemperature. Simultaneously, each of the main pumping cell 44, thecontrolling oxygen partial pressure-detecting cell 50, and the measuringpumping cell 64 is also heated to a predetermined temperature andmaintained at that temperature.

[0109] Next, the operation of the gas sensor 10A according to the firstembodiment will be explained. At first, the forward end of the gassensor 10A is disposed in the external space. Accordingly, themeasurement gas is introduced into the first chamber 18 under thepredetermined diffusion resistance via the first diffusionrate-determining section 26 (slits 30, 32). The measurement gas, whichhas been introduced into the first chamber 18, is subjected to thepumping action for oxygen, caused by applying the predetermined pumpingvoltage Vp1 between the outer pumping electrode 42 and the inner pumpingelectrode 40 which construct the main pumping cell 44. The partialpressure of oxygen is controlled to have a predetermined value, forexample, 10⁻⁷ atm. The control is performed by the aid of the feedbackcontrol system 52.

[0110] The first diffusion rate-determining section 26 serves to limitthe amount of diffusion and inflow of oxygen in the measurement gas intothe measuring space (first chamber 18) when the pumping voltage Vp1 isapplied to the main pumping cell 44 so that the current flowing throughthe main pumping cell 44 is suppressed.

[0111] In the first chamber 18, a state of partial pressure of oxygen isestablished, in which NOx in the atmosphere is not reduced by the innerpumping electrode 40 in an environment of being heated by the externalmeasurement gas and being heated by the heater 80. For example, acondition of partial pressure of oxygen is formed, in which the reactionof NO→½N₂+½O₂ does not occur, because of the following reason. That is,if NOx in the measurement gas (atmosphere) is reduced in the firstchamber 18, it is impossible to accurately measure NOx in the secondchamber 20 disposed at the downstream stage. In this context, it isnecessary to establish a condition in the first chamber 18 in which NOxis not reduced by the component which participates in reduction of NOx(in this case, the metal component of the inner pumping electrode 40).Specifically, as described above, such a condition is achieved by using,for the inner pumping electrode 40, the material having a low ability toreduce NOx, for example, an alloy of Au and Pt.

[0112] The gas in the first chamber 18 is introduced into the secondchamber 20 under the predetermined diffusion resistance via the seconddiffusion rate-determining section 28. The gas, which has beenintroduced into the second chamber 20, is subjected to the pumpingaction for oxygen, caused by applying the voltage Vp3 between thereference electrode 48 and the auxiliary pumping electrode 70 whichconstitute the auxiliary pumping cell 72 to make fine adjustment so thatthe partial pressure of oxygen has a constant and low value of partialpressure of oxygen.

[0113] The second diffusion rate-determining section 28 serves to limitthe amount of diffusion and inflow of oxygen in the measurement gas intothe measuring space (second chamber 20) when the voltage Vp3 is appliedto the auxiliary pumping cell 72 so that the pumping current Ip3 flowingthrough the auxiliary pumping cell 72 is suppressed, in the same manneras performed by the first diffusion rate-determining section 26.

[0114] The measurement gas, which has been controlled for the partialpressure of oxygen in the second chamber 20 as described above, isintroduced into the detecting electrode 60 under the predetermineddiffusion resistance via the third diffusion rate-determining section62.

[0115] When it is intended to control the partial pressure of oxygen inthe atmosphere in the first chamber 18 to have a low value of thepartial pressure of oxygen which does not substantially affect themeasurement of NOx, by operating the main pumping cell 44, in otherwords, when the pumping voltage Vp1 of the variable power source 46 isadjusted by the aid of the feedback control system 52 so that thevoltage V1 detected by the controlling oxygen partial pressure-detectingcell 50 is constant, if the oxygen concentration in the measurement gasgreatly changes, for example, in a range of 0 to 20%, then therespective partial pressures of oxygen in the atmosphere in the secondchamber 20 and in the atmosphere in the vicinity of the detectingelectrode 60 slightly change in ordinary cases. This phenomenon iscaused probably because of the following reason. That is, when theoxygen concentration in the measurement gas increases, the distributionof the oxygen concentration occurs in the widthwise direction and in thethickness direction in the first chamber 18. The distribution of theoxygen concentration changes depending on the oxygen concentration inthe measurement gas.

[0116] However, in the case of the gas sensor 10A according to the firstembodiment, the auxiliary pumping cell 72 is provided for the secondchamber 20 so that the partial pressure of oxygen in its internalatmosphere always has a constant low value of the partial pressure ofoxygen. Accordingly, even when the partial pressure of oxygen in theatmosphere introduced from the first chamber 18 into the second chamber20 changes depending on the oxygen concentration in the measurement gas,the partial pressure of oxygen in the atmosphere in the second chamber20 can be always made to have a constant low value, owing to the pumpingaction performed by the auxiliary pumping cell 72. As a result, thepartial pressure of oxygen can be controlled to have a low value atwhich the measurement of NOx is not substantially affected.

[0117] NOx in the measurement gas introduced into the detectingelectrode 60 is reduced or decomposed around the detecting electrode 60.Thus, for example, a reaction of NO→½N₂+½O₂ is allowed to occur. In thisprocess, a predetermined voltage Vp2, for example, 430 mV (700° C.) isapplied between the detecting electrode 60 and the reference electrode48 which construct the measuring pumping cell 64, in a direction to pumpout the oxygen from the second chamber 20 to the referencegas-introducing space 16.

[0118] Therefore, the pumping current Ip2 flowing through the measuringpumping cell 64 has a value which is proportional to a sum of the oxygenconcentration in the atmosphere introduced into the second chamber 20,i.e., the oxygen concentration in the second chamber 20 and the oxygenconcentration produced by reduction or decomposition of NOx by the aidof the detecting electrode 60.

[0119] In this embodiment, the oxygen concentration in the atmosphere inthe second chamber 20 is controlled to be constant by means of theauxiliary pumping cell 72. Accordingly, the pumping current Ip2 flowingthrough the measuring pumping cell 64 is proportional to the NOxconcentration. The NOx concentration corresponds to the amount ofdiffusion of NOx limited by the third diffusion rate-determining section62. Therefore, even when the oxygen concentration in the measurement gasgreatly changes, it is possible to accurately measure the NOxconcentration, based on the use of the measuring pumping cell 64 by theaid of the ammeter 68.

[0120] According to the fact described above, almost all of the pumpingcurrent value Ip2 obtained by operating the measuring pumping cell 64represents the amount brought about by the reduction or decomposition ofNOx. Accordingly, the obtained result does not depend on the oxygenconcentration in the measurement gas.

[0121] In the meantime, in ordinary cases, the shift amount increasesproportionally from the stage at which the ratio between the dynamicpressure and the static pressure (dynamic pressure/static pressure)exceeds about 25% (see FIG. 34). However, in the gas sensor 10Aaccording to the first embodiment, the length ta in the verticaldirection, which is the certain factor for forming the cross-sectionalconfiguration of the first diffusion rate-determining section 26 (slits30, 32), is not more than 10 μm.

[0122] The limiting current value Ip1 in the main pumping cell 44 isapproximated by the following theoretical expression for the limitingcurrent.

Ip 1≈(4F/RT)×D×(S/L)×(POe−POd)

[0123] In the expression, F represents the Faraday constant (=96500A/sec), R represents the gas constant (=82.05 cm³·atm/mol·K), Trepresents the absolute temperature (K), D represents the diffusioncoefficient (cm²/sec), S represents the cross-sectional area (cm²) ofthe first diffusion rate-determining section 26 (slit 30 or 32), Lrepresents the passage length (cm) of the first diffusionrate-determining section 26 (slit 30 or 32), POe represents the partialpressure of oxygen (atm) in the external space, and POd represents thepartial pressure of oxygen (atm) in the first chamber 18.

[0124] In the gas sensor 10A according to the first embodiment, there isdefined the formation factor for the cross-sectional area S of the firstdiffusion rate-determining section 26 (slit 30 or 32) in the theoreticallimiting current expression. Especially, it is defined that the certainfactor of the dimension for forming the cross-sectional area S, i.e.,the length in the vertical direction in this embodiment is not more than10 μm.

[0125] Accordingly, the pulsation (=dynamic pressure) of the exhaust gaspressure is attenuated by the wall resistance of the first diffusionrate-determining section 26. Specifically, the attenuation is effectedup to the level at which the ratio between the dynamic pressure and thestatic pressure (dynamic pressure/static pressure) is not more than 25%.Therefore, it is possible to effectively suppress the shift-upphenomenon of the sensor output (the pumping current value Ip2concerning the measuring pumping cell or the value of the pumpingcurrent Ip1 flowing through the main pumping cell) which would beotherwise caused by the fluctuation of the dynamic pressure.

[0126] Two illustrative experiments (conveniently referred to as “firstand second illustrative experiments”) will now be described. In thefirst illustrative experiment, the measurement was made for the way ofchange of the sensor output obtained when the oxygen concentration inthe measurement gas was changed concerning Working Example andComparative Example. In the second illustrative experiment, themeasurement was made for the way of change of the sensor output obtainedwhen the NOx concentration in the measurement gas was changed.

[0127] The following measurement condition was adopted. That is, adiesel engine of 2.5 L was used as an engine, the number of revolutionwas 1000 to 4000 rpm, and the engine load was 5 to 20 kgm. The number ofrevolution, the engine load, and the EGR opening degree wereappropriately changed to measure the fluctuation of the sensor outputobtained under the respective conditions as well.

[0128] Working Example is illustrative of the case in which the firstdiffusion rate-determining section 26 was constructed by upper and lowertwo laterally extending slits 30, 32 (length in the lateral direction: 2mm×length in the vertical direction: not more than 10 μm) as shown inFIGS. 3A, 3B, and 4 in the same manner as in the gas sensor 10Aaccording to the first embodiment. Comparative Example is illustrativeof the case in which the first diffusion rate-determining section 26 wasconstructed by a single slit 100 (length in the lateral direction: 0.2mm×length in the vertical direction: 0.2 mm) as shown in FIGS. 5A, 5B,and 6 concerning the gas sensor 10A according to the first embodiment.

[0129] Experimental results obtained in the first and secondillustrative experiments are shown in FIGS. 7 and 8 (ComparativeExample) and in FIGS. 9 and 10 (Working Example). It is understood thatin Comparative Example as shown in FIGS. 7 and 8, the sensor outputfluctuates by changing the measurement condition, and the fluctuation ofthe sensor output is conspicuous especially when the engine load ishigh.

[0130] The above result is obtained probably because of the followingreason. That is, as shown in FIGS. 11A and 11B to depict waveforms, thefluctuation of the exhaust gas pressure in the vicinity of thegas-introducing port is approximately the same as the fluctuation of theexhaust gas pressure in the vicinity of the inlet of the first chamber18, and the fluctuation of the exhaust gas pressure associated with thechange in measurement condition directly affects the sensor output.

[0131] On the other hand, in Working Example, the sensor output does notfluctuates as shown in FIGS. 9 and 10 even when the measurementcondition is changed. It is possible to obtain the sensor outputdepending on the change in oxygen concentration and NOx concentration ata high degree of accuracy. This is probably because of the followingreason. That is, as shown in FIGS. 12A and 12B to depict waveforms, thefluctuation of the exhaust gas pressure in the vicinity of thegas-introducing port is attenuated by the wall resistance of the firstdiffusion rate-determining section 26. Therefore, the fluctuation of theexhaust gas pressure in the vicinity of the inlet of the first chamber18 is smaller than the fluctuation of the exhaust gas pressure in thevicinity of the gas-introducing port.

[0132] As described above, the gas sensor 10A according to the firstembodiment makes it possible to avoid the influence of the pulsation ofthe exhaust gas pressure generated in the measurement gas. Thus, it ispossible to improve the measurement accuracy obtained on the measuringpumping cell 64.

[0133] Next, explanation will be made with reference to FIGS. 13A to 30for several modified embodiments of the gas sensor 10A according to thefirst embodiment, namely modified embodiments principally concerning theshape of the first diffusion rate-determining section 26 and the seconddiffusion rate-determining section 28. In FIGS. 13A to 30, the electriccircuit system is omitted from the illustration in order to avoidcomplicated drawings. Components or parts corresponding to those shownin FIG. 1 are designated by the same reference numerals, duplicateexplanation of which will be omitted.

[0134] At first, as shown in FIGS. 13A, 13B, and 14, a gas sensor 10Aaaccording to the first modified embodiment differs in that the first andsecond diffusion rate-determining sections 26, 28 are formed by singlelaterally extending slits 110, 112 respectively.

[0135] Specifically, the first diffusion rate-determining section 26includes the slit 110 having a laterally extending aperture formed at afront end portion of the second spacer layer 12 e to make contact withthe upper surface of the first solid electrolyte layer 12 d, theaperture being formed to extend with an identical aperture width up tothe first chamber 18. The second diffusion rate-determining section 28includes the slit 112 having an aperture formed at a terminal endportion of the first chamber 18 of the second spacer layer 12 e to makecontact with the upper surface of the first solid electrolyte layer 12d, the aperture being formed to extend with an identical aperture widthup to the second chamber 20. In the first modified embodiment, each ofthe slits 110, 112 has approximately the same cross-sectionalconfiguration, in which the length ta in the vertical direction is notmore than 10 μm, and the length tb in the lateral direction is about 2mm.

[0136] Next, as shown in FIGS. 15A, 15B, and 16, a gas sensor 10Abaccording to the second modified embodiment differs in that the firstand second diffusion rate-determining sections 26, 28 are formed bysingle laterally extending wedge-shaped slits 114, 116 respectively.

[0137] Specifically, the first diffusion rate-determining section 26includes the wedge-shaped slit 114 having a laterally extending apertureformed at a front end portion of the second spacer layer 12 e to makecontact with the upper surface of the first solid electrolyte layer 12d, the aperture being formed to extend with an aperture width (width inthe vertical direction) gradually enlarged toward the first chamber 18.The second diffusion rate-determining section 28 includes thewedge-shaped slit 116 having a laterally extending aperture formed at aterminal end portion of the first chamber 18 of the second spacer layer12 e to make contact with the upper surface of the first solidelectrolyte layer 12 d, the aperture being formed to extend with anaperture width gradually enlarged toward the second chamber 20.

[0138] In the second modified embodiment, the minimum aperture at thefront end of each of the wedge-shaped slits 114, 116 has approximatelythe same cross-sectional configuration, in which the length ta in thevertical direction is not more than 10 μm, and the length tb in thelateral direction is about 2 mm.

[0139] Next, as shown in FIGS. 17A, 17B, and 18, a gas sensor 10Acaccording to the third modified embodiment differs in that the firstdiffusion rate-determining section 26 is formed by three laterallyextending slits 118 a, 118 b, 118 c which are disposed in parallel toone another, and the second diffusion rate-determining section 28 isformed by a single laterally extending slit 120.

[0140] Specifically, the first diffusion rate-determining section 26includes the three slits 118 a, 118 b, 118 c having three laterallyextending apertures formed in parallel to one another at front endportions of the second spacer layer 12 e to make contact with the uppersurface of the first solid electrolyte layer 12 d, each of the aperturesbeing formed to extend with an identical aperture width up to the firstchamber 18. The second diffusion rate-determining section 28 includesthe single slit 120 having a single laterally extending aperture formedat a terminal end portion of the first chamber 18 of the second spacerlayer 12 e to make contact with the upper surface of the first solidelectrolyte layer 12 d, the aperture being formed to extend with anidentical aperture width up to the second chamber 20. In the thirdmodified embodiment, each of the slits 118 a, 118 b, 118 c, 120 has thelength ta in the vertical direction which is not more than 10 μm.

[0141] Next, as shown in FIGS. 19A, 19B, and 20, a gas sensor 10Adaccording to the fourth modified embodiment differs in that a spacesection 122 and a buffering space 124 are provided in series between thegas-introducing port 22 and the first diffusion rate-determining section26, a front aperture of the space section 122 constitutes thegas-introducing port 22, and a fourth diffusion rate-determining section126 for giving a predetermined diffusion resistance to the measurementgas is provided between the space section 122 and the buffering space124.

[0142] Each of the first diffusion rate-determining section 26 and thesecond diffusion rate-determining section 28 is formed by two laterallyextending slits 30, 32, 34, 36, in the same manner as in the gas sensor10A according to the first embodiment.

[0143] The fourth diffusion rate-determining section 126 includes a slit128 having a laterally extending aperture formed at a terminal endportion of the space section 122 of the second spacer layer 12 e to makecontact with the lower surface of the second solid electrolyte layer 12f, the aperture being formed to extend with an identical aperture widthup to the buffering space 124. The fourth diffusion rate-determiningsection 126 further includes a slit 130 having a laterally extendingaperture formed at a terminal end portion of the space section 122 ofthe second spacer layer 12 e to make contact with the upper surface ofthe first solid electrolyte layer 12 d, the aperture being formed toextend with an identical aperture width up to the buffering space 124.

[0144] Next, as shown in FIGS. 21A, 21B, and 22, a gas sensor 10Aeaccording to the fifth modified embodiment differs in that each of thefirst and second diffusion rate-determining sections 26, 28 is formed bya single vertically extending slit 132, 134.

[0145] Specifically, the first diffusion rate-determining section 26includes the slit 132 having a vertically extending aperture formed at afront end portion of the second spacer layer 12 e approximately at thecenter in the widthwise direction thereof, the aperture being formed toextend with an identical aperture width up to the first chamber 18. Thesecond diffusion rate-determining section 28 includes the slit 134having a vertically extending aperture formed at a terminal end portionof the first chamber 18 of the second spacer layer 12 e approximately atthe center in the widthwise direction thereof, the aperture being formedto extend with an identical aperture width up to the second chamber 20.In the fifth modified embodiment, each of the slits 132, 134 hasapproximately the same cross-sectional configuration, in which thelength tc in the vertical direction is the same as the thickness of thesecond spacer layer 12 e, and the length td in the lateral direction isnot more than 10 μm.

[0146] Next, as shown in FIGS. 23A, 23B, and 24, a gas sensor 10Afaccording to the sixth modified embodiment differs in that the first andsecond diffusion rate-determining sections 26, 28 are formed by singlevertically extending wedge-shaped slits 136, 138 respectively.

[0147] Specifically, the first diffusion rate-determining section 26includes the wedge-shaped slit 136 having a vertically extendingaperture formed at a front end portion of the second spacer layer 12 eapproximately at the center in the widthwise direction thereof, theaperture being formed to extend with an aperture width (width in thelateral direction) gradually enlarged toward the first chamber 18. Thesecond diffusion rate-determining section 28 includes the wedge-shapedslit 138 having a vertically extending aperture formed at a terminal endportion of the first chamber 18 of the second spacer layer 12 eapproximately at the center in the widthwise direction thereof, theaperture being formed to extend with an aperture width (width in thelateral direction) gradually enlarged toward the second chamber 20.

[0148] In the sixth modified embodiment, the minimum aperture at thefront end of each of the wedge-shaped slits 136, 138 has approximatelythe same cross-sectional configuration, in which the length tc in thevertical direction is the same as the thickness of the second spacerlayer 12 e, and the length td in the lateral direction is not more than10 μm.

[0149] Next, as shown in FIGS. 25A, 25B, and 26, a gas sensor 10Agaccording to the seventh modified embodiment differs in that the firstdiffusion rate-determining section 26 is formed by a slit 140 having asubstantially hourglass-shaped planar configuration, and the seconddiffusion rate-determining section 28 is formed by a single verticallyextending slit 142.

[0150] Specifically, the first diffusion rate-determining section 26includes the substantially hourglass-shaped slit 140 starting from alaterally extending aperture formed at a front end portion of the secondspacer layer 12 e, the aperture having its aperture width (width in thelateral direction) gradually decreasing toward the approximate center ofthe first diffusion rate-determining section 26 in the depth directionto form a vertically extending slit 144, in which the aperture width(width in the lateral direction) of the slit 144 gradually increasestoward the first chamber 18 to form the substantially hourglass-shapedslit 140.

[0151] On the other hand, the second diffusion rate-determining section28 includes the slit 142 having a vertically extending aperture formedat a terminal end portion of the first chamber 18 of the second spacerlayer 12 e approximately at the center in the widthwise directionthereof, the aperture being formed to extend with an identical aperturewidth up to the second chamber 20.

[0152] In the seventh modified embodiment, the minimum aperture (slit144) of the hourglass-shaped slit 140 for constructing the firstdiffusion rate-determining section 26 has approximately the samecross-sectional configuration as that of the slit 142 for constructingthe second diffusion rate-determining section 28, in which the length tcin the vertical direction is the same as the thickness of the secondspacer layer 12 e, and the length td in the lateral direction is notmore than 10 μm.

[0153] Next, as shown in FIGS. 27A, 27B, and 28, a gas sensor 10Ahaccording to the eighth modified embodiment differs in that the firstdiffusion rate-determining section 26 is formed by five verticallyextending slits 146 a to 146 e which are disposed in parallel to oneanother, and the second diffusion rate-determining section 28 is formedby a single vertically extending slit 148.

[0154] Specifically, the first diffusion rate-determining section 26includes the five slits 146 a to 146 e having five vertically extendingapertures formed in parallel to one another at terminal end portions ofthe second spacer layer 12 e, each of the apertures being formed toextend with an identical aperture width up to the first chamber 18. Thesecond diffusion rate-determining section 28 includes the single slit148 having a vertically extending aperture formed at a terminal endportion of the first chamber 18 of the second spacer layer 12 eapproximately at the center in the widthwise direction thereof, theaperture being formed to extend with an identical aperture width up tothe second chamber 20. In the eighth modified embodiment, the length tdin the lateral direction of each of the slits 146 a to 146 e, 148 is notmore than 10 μm.

[0155] Next, as shown in FIGS. 29A, 29B, and 30, a gas sensor 10Aiaccording to the ninth modified embodiment differs in that a spacesection 122 and a buffering space 124 are provided in series between thegas-introducing port 22 and the first diffusion rate-determining section26, a front aperture of the space section 122 constitutes thegas-introducing port 22, and a fourth diffusion rate-determining section126 for giving a predetermined diffusion resistance to the measurementgas is provided between the space section 122 and the buffering space124.

[0156] Each of the first diffusion rate-determining section 26 and thesecond diffusion rate-determining section 28 is formed by a singlelaterally extending slit 132, 134, in the same manner as in the gassensor 10Ae according to the fifth modified embodiment (see FIGS. 21A,21B, and 22).

[0157] The fourth diffusion rate-determining section 126 includes a slit150 having a vertically extending aperture formed at a terminal endportion of the space section 122 of the second spacer layer 12 eapproximately at the center in the widthwise direction thereof, theaperture being formed to extend with an identical aperture width up tothe buffering space 124.

[0158] The gas sensors 10Aa to 10Ai according to the first to ninthmodified embodiments make it possible to avoid the influence of thepulsation of the exhaust gas pressure generated in the measurement gas,in the same manner as the gas sensor 10A according to the firstembodiment. Thus, it is possible to improve the measurement accuracyobtained on the measuring pumping cell 64.

[0159] Especially, the gas sensors 10Ad and 10Ai according to the fourthand ninth modified embodiments include the buffering space 124 providedat the upstream stage of the first diffusion rate-determining section26. Usually, the oxygen suddenly enters the sensor element 14 via thegas-introducing port 22 due to the pulsation of the exhaust gas pressurein the external space. However, in this arrangement, the oxygen from theexternal space does not directly enter the processing space, but itenters the buffering space 124 disposed at the upstream stage thereof.In other words, the sudden change in oxygen concentration, which iscaused by the pulsation of the exhaust gas pressure, is counteracted bythe buffering space 124. Thus, the influence of the pulsation of theexhaust gas pressure on the first chamber 18 is in an almost negligibledegree.

[0160] As a result, the correlation is improved between theoxygen-pumping amount effected by the main pumping cell 44 for the firstchamber 18 and the oxygen concentration in the measurement gas. It ispossible to improve the measurement accuracy obtained by the measuringpumping cell 64. Simultaneously, for example, it is possible toconcurrently use the first chamber 18 as a sensor for determining theair-fuel ratio.

[0161] The gas sensors 10Ad and 10Ai according to the fourth and ninthmodified embodiment described above include the space section 122 andthe buffering space 124 which are provided in series between thegas-introducing port 22 and the first diffusion rate-determining section26, and the front aperture of the space section 122 is used to form thegas-introducing port 22. The space section 122 functions as theclogging-preventive section for avoiding the clogging of particles (forexample, soot and oil combustion waste) produced in the measurement gasin the external space, which would be otherwise caused in the vicinityof the inlet of the buffering space 124. Accordingly, it is possible tomeasure the NOx component more accurately by using the measuring pumpingcell 64. Further, it is possible to maintain a highly accurate stateover a long period of time.

[0162] The gas sensor 10A according to the first embodiment and the gassensors 10Aa to 10Ad according to the first to fourth modifiedembodiments described above have their first and second diffusionrate-determining sections 26, 28 each of which is constructed by thelaterally extending slit. The gas sensors 10Ae to 10Ai according to thefifth to ninth modified embodiments have their first and seconddiffusion rate-determining sections 26, 28 each of which is constructedby the vertically extending slit. However, for example, the firstdiffusion rate-determining section 26 may be constructed by a laterallyextending slit, and the second diffusion rate-determining section 28 maybe constructed by a vertically extending slit. Alternatively, it is alsoallowable to adopt an arrangement in which the first and seconddiffusion rate-determining sections 26, 28 are constructed in an inversemanner of the above.

[0163] It is also allowable that the shape of each of the first andsecond diffusion rate-determining sections 26, 28 is not the slit-shapedconfiguration provided that the certain factor for constructing thecross-sectional area is not more than 10 μm. For example, an equivalenteffect can be obtained such that a sublimable fiber is embedded, and acylindrical diffusion rate-determining section having a diameter of notmore than 10 μm is constructed after the sintering.

[0164] Next, a gas sensor 10B according to the second embodiment will beexplained with reference to FIG. 31. Components or parts correspondingto those shown in FIG. 2 are designated by the same reference numerals,duplicate explanation of which will be omitted.

[0165] As shown in FIG. 31, the gas sensor 10B according to the secondembodiment is constructed in approximately the same manner as the gassensor 10A according to the first embodiment (see FIG. 2). However, theformer is different from the latter in that a measuring oxygen partialpressure-detecting cell 160 is provided in place of the measuringpumping cell 64.

[0166] The measuring oxygen partial pressure-detecting cell 160comprises a detecting electrode 162 formed on an upper surface portionfor forming the second chamber 20, of the upper surface of the firstsolid electrolyte layer 12 d, the reference electrode 48 formed on thelower surface of the first solid electrolyte layer 12 d, and the firstsolid electrolyte layer 12 d interposed between the both electrodes 162,48.

[0167] In this embodiment, an electromotive force (electromotive forceof an oxygen concentration cell) V2 corresponding to the difference inoxygen concentration between the atmosphere around the detectingelectrode 162 and the atmosphere around the reference electrode 48 isgenerated between the reference electrode 48 and the detecting electrode162 of the measuring oxygen partial pressure-detecting cell 160.

[0168] Therefore, the partial pressure of oxygen in the atmospherearound the detecting electrode 162, in other words, the partial pressureof oxygen defined by oxygen produced by reduction or decomposition ofthe measurement gas component (NOx) is detected as a voltage value V2 bymeasuring the electromotive force (voltage) V2 generated between thedetecting electrode 162 and the reference electrode 48 by using avoltmeter 164.

[0169] Also in the gas sensor 10B according to the second embodiment,the pulsation (=dynamic pressure) of the exhaust gas pressure isattenuated by the wall resistance of the first diffusionrate-determining section 26. Therefore, it is possible to effectivelysuppress the shift-up phenomenon of the sensor output (pumping currentvalue obtained by using the measuring pumping cell) which would beotherwise caused by the fluctuation of the dynamic pressure. As aresult, it is possible to avoid the influence of the pulsation of theexhaust gas pressure generated in the measurement gas. It is possible toimprove the measurement accuracy concerning the measuring oxygen partialpressure-detecting cell 160.

[0170] The arrangements of the gas sensors 10Aa to 10Ai according to thefirst to ninth modified embodiments can be also adopted to the gassensor 10B according to the second embodiment.

[0171] The gas sensors 10A and 10B according to the first and secondembodiments described above (including the respective modifiedembodiments) are directed to oxygen and NOx as the measurement gascomponent to be measured. However, the present invention is alsoeffectively applicable to the measurement of bound oxygen-containing gascomponents such as H₂O and CO₂ other than NOx, in which the measurementis affected by oxygen existing in the measurement gas.

[0172] For example, the present invention is also applicable to gassensors which are constructed to pump out O₂ produced by electrolysis ofCO₂ or H₂O by using the oxygen pump, and to gas sensors in which H₂produced by electrolysis of H₂O is pumping-processed by using a protonion-conductive solid electrolyte.

[0173] It is a matter of course that the gas sensor and the nitrogenoxide sensor according to the present invention are not limited to theembodiments described above, which may be embodied in other variousforms without deviating from the gist or essential characteristics ofthe present invention.

[0174] As described above, according to the gas sensor and the nitrogenoxide sensor concerning the present invention, it is possible to avoidthe influence of the pulsation of the exhaust gas pressure generated inthe measurement gas, and it is possible to improve the measurementaccuracy obtained on the detecting electrode.

What is claimed is:
 1. A gas sensor for measuring an amount of ameasurement gas component contained in a measurement gas existing in anexternal space, said gas sensor at least comprising: a substratecomposed of a solid electrolyte to make contact with the external space;an internal space formed at the inside of said substrate; agas-introducing port for introducing measurement gas from the externalspace into said internal space; a clogging-preventative section providedin series between said gas-introducing port and said internal space, afront aperture of said clogging-preventive section defining saidgas-introducing port; a diffusion rate-determining slit passage providedin series between said clogging-preventative section and said internalspace, for introducing measurement gas from the external space into saidinternal space under a predetermined diffusion resistance, said slitpassage having, when viewed in a plane substantially perpendicular to alongitudinal extension axis thereof, two dimensions defining across-sectional area, wherein the cross-sectional area of said slitpassage is less than that of said clogging-preventative section and atleast one dimension of said slit passage is not more than 10 microns;and pumping means including an inner pumping electrode and an outerpumping electrode formed at the inside and outside of said internalspace respectively, for pumping-processing oxygen contained in themeasurement gas introduced from the external space, on the basis of acontrol voltage applied between said electrodes.
 2. The gas sensoraccording to claim 1, wherein said oxygen contained in the measurementgas introduced from the external space into said internal space ispumping-processed by using said pumping means to such an extent that apartial pressure of oxygen in said internal space has a predeterminedvalue at which a predetermined gas component in the measurement gas isnot decomposable.
 3. The gas sensor according to claim 2, furthercomprising: measuring pumping means for decomposing the predeterminedgas component contained in the measurement gas after beingpumping-processed by said pumping means, by means of catalytic actionand/or electrolysis, and pumping processing oxygen produced by saiddecomposition, wherein the predetermined gas component contained in themeasurement gas is measured on the basis of a pumping current flowingthrough said measuring pumping means in accordance with said pumpingprocess effected by said measuring pumping means.
 4. The gas sensoraccording to claim 2, further comprising: oxygen partialpressure-detecting means for decomposing the predetermined gas componentcontained in the measurement gas after being pumping-processed by saidpumping means, by means of catalytic action, and generating anelectromotive force corresponding to a difference between an amount ofoxygen produced by said decomposition and an amount of oxygen containedin a reference gas, wherein the predetermined gas component contained inthe measurement gas is measured on the basis of said electromotive forcedetected by said oxygen partial pressure-detecting means.
 5. The gassensor according to claim 1, wherein said substrate is substantiallyplanar and said at least one dimension of said slit passage extendsgenerally perpendicular to the plane of said substrate.
 6. The gassensor according to claim 1, wherein said substrate is substantiallyplanar and said at least one dimension of said slit passage extendsgenerally within the plane of said substrate.